The invention relates to an image sensor comprising a semiconductor body which is provided, at a surface, with electrodes, each electrode being combined with the semiconductor body and an intermediate dielectric so as to form a MOS capacitor, which electrodes have a portion which is thinner than a surrounding zone, a photosensitive region in the semiconductor body being situated below each electrode, which photosensitive region is capable of absorbing electromagnetic radiation and converting said electromagnetic radiation to electric charge.
The invention also relates to a method of manufacturing an image sensor comprising a semiconductor body which is provided, at a surface, with electrodes, each electrode being combined with the semiconductor body and an intermediate dielectric so as to form a MOS capacitor, second electrodes having a portion which is thinner than a surrounding zone, a photosensitive region in the semiconductor body being situated below each electrode, which photosensitive region is capable of absorbing electromagnetic radiation and converting said radiation to electric charge, first electrodes being formed from a first layer of polysilicon, and insulation being provided between the first electrodes and the second electrodes.
A method of manufacturing such an image sensor is known from U.S. Pat. No. 5,210,049. In the known method, an image sensor is manufactured, which image sensor comprises, inter alia, a matrix of pixels and a CCD shift register. A pixel comprises a photocapacitor, a transfer gate and an overflow gate. Below each electrode of the photocapacitor there is a photosensitive region in the semiconductor body, which photosensitive region absorbs electromagnetic radiation and converts it to electric charge. Said electric charge can be displaced via the transfer gate and read via a shift register. If too much charge is converted in a pixel, a part of the charge is removed via the overflow gate. To increase the light sensitivity of the photocapacitor, the light-receiving electrode is locally made thinner. The absorption of short-wave electromagnetic radiation, in particular the blue light of the visible spectrum, is substantially reduced thereby.
In the known method, a conductive region is formed in a semiconductor body. dielectric is provided on the semiconductor body. On the dielectric, a first layer of conductive polycrystalline Si is applied above the conductive region, from which the transfer gate and the overflow gate are formed. After the application of insulating material, a second layer of conductive polycrystalline Si is applied above the conductive region, from which the electrodes of the photocapacitors and the electrodes of the shift register are simultaneously formed. The uppermost electrode of the photocapacitor is locally reduced in thickness to a value at which the absorption and losses caused by interference of incident light are reduced, so that the amount of light reaching the conductive region and the region of the semiconductor below the conductive region increases. The uppermost electrode of the photocapacitor is locally reduced in thickness by means of a pattern in a resist layer and by etching the second conductive polysilicon layer in the apertures of the resist pattern.
A drawback of the known image sensor resides in that the photocapacitors, the transfer gates and the overflow transfer gates take up a comparatively large Si semiconductor surface. The photosensitive part formed by the photocapacitors is only a small part of the overall surface of the image sensor. The sensitivity of the image sensor to, in particular, short-wave electromagnetic radiation is small.
An additional disadvantage resides in that the thickness of the thin portion of the electrodes of the photocapacitors is difficult to control. As a result of the topography of the first transfer gate and the overflow gate, on top of which the second polysilicon layer is deposited, the step coverage depends substantially upon the space between the transfer gate and the overflow gate and the thickness of the first polysilicon layer. The second polysilicon layer is a very thick layer having a thickness of several microns. The thickness of the thick layer can be locally reduced to 50 nm by subjecting it to an etching operation, however, such an etching operation is poorly reproducible and leads to the introduction of a large spread. As the thickness of the thin polysilicon is not uniform, the sensitivity of the pixels varies substantially.
It is an object of the invention to provide an image sensor of the type described in the opening paragraph, which image sensor has a greater sensitivity to electromagnetic radiation, in particular short-wave electromagnetic radiation.
A further object of the invention is to provide a method of manufacturing an image sensor of the type described in the opening paragraph, which image sensor has a greater sensitivity, can be manufactured more readily and is more reliable.
In the device in accordance with the invention, this object is achieved in that the MOS capacitors are arranged next to each other in a matrix array, the electrodes in a row being interconnected and electrically contacting each other, and the electrodes in a column being separated only by electrically insulating material.
As the MOS capacitors are interconnected in a row and, in the column direction are very closely spaced, substantially the entire photosensitive surface is covered with electrodes. The electrodes comprise a comparatively large thinner portion in order to absorb more electromagnetic radiation in the photosensitive regions, which electromagnetic radiation is converted to electric charge. The photosensitivity to, in particular, short-wave electromagnetic radiation is improved substantially by increasing the photosensitive surface. By means of the electrode of a MOS capacitor, the charge is collected below the electrode. A larger photosensitive surface does not only increase the sensitivity but also the charge-storage capacity of a pixel. By virtue thereof, the signal-to-noise ratio of the image sensor is improved, as a result of which, ultimately, the image can be sharper and brighter.
In order to be able to sufficiently rapidly read the charge below each electrode of the MOS capacitor using a clock signal of, for example, 1 MHz, the delay caused by the RC time may not become excessively long. The interconnected electrodes in a row electrically contact each other and determine the resistance. By providing the electrodes with thicker portions around the thinner portions, the resistance is reduced substantially. It is very favorable that, by means of said thick portions of the electrodes, it becomes possible to just reach the clock rate, and the remaining surface of the electrodes is very thin in order to allow as much electromagnetic radiation as possible to pass to the photosensitive regions.
Advantageously, the locally thinner portion of each electrode is centered in the relevant electrode so as to preclude, to the extent possible, reflections of light at the edges between different media and any differences in thickness at the edges of the polysilicon electrodes. In addition, thicker edge portions of the electrodes are very favorable because, in general, the current densities that can be attained along edges are larger than in the center, leading to a reduction of the resistance of the electrodes. In addition, it is advantageous if as much as possible of the electromagnetic radiation lands on the photosensitive regions, i.e. the so-called channels for the charge transport. The photosensitive regions are bounded in the horizontal directions by zones of a different doping type. Depletion regions develop between the two doping regions. The zones and the depletion regions at the edges of the electrodes are less suitable for converting electromagnetic radiation to electric charge. Therefore, it is favorable for the locally thinner portion of each electrode to be centered.
Favorably, the locally thinner portion of each electrode covers at least 25% of each electrode surface. The amount of charge converted per pixel in the course of a certain integration period is typically several ten thousand times larger than the charge caused by the dark current. To attain a good signal-to-noise ratio, it is thus advantageous for the thin portion of the electrodes to be as large as possible. After each integration period, the charge packets are transported, in the CCD manner, through the channels towards a horizontal readout register. During the transport of the charge packets through the channels towards the horizontal read-out register, also light is absorbed and converted. This leads to the formation of noise on the signal. Consequently, reading must take place rapidly, for example at a clock frequency of several MHz. The zone surrounding the thin portion of the electrodes preferably covers only such a part of the surface of the electrodes as is necessary to achieve a sufficiently low resistance, while the remaining surface of the electrodes is thin. In general, the read-out velocity is sufficient if the surrounding zones of the electrodes cover up to 25% of the surface area.
Advantageously, the charge below an electrode can be displaced during a clock signal on the same electrode. In a Frame Transfer (FT) image sensor, an image is stored in the imaging portion of the CCD sensor during a certain integration time, and, subsequently, said image is rapidly transported by a clock signal on the electrodes to a memory portion of the CCD sensor where it is read out line by line. In the FT sensor, the charge-sensitive region also forms the charge-transfer layer. The gate extends throughout the region, resulting in an optimum charge control. The object of the invention as regards the method is achieved, in accordance with the invention, in that a second layer of polysilicon is reduced in thickness to substantially the same thickness as that of the first polysilicon layer, and thinner portions are formed in all electrodes by etching the polysilicon.
A very short distance between the first and second electrodes can be achieved, for example, by thermal oxidation of the first polysilicon electrodes, resulting in the formation of a thin insulation layer of SiO2. By reducing the thickness of the second layer of conductive polysilicon so as to be the same as the thickness of the polysilicon layer that is applied first, a matrix of very closely spaced substantially equally thick electrodes is obtained. As there are hardly any differences in topography, it is possible, using a resist pattern wherein apertures are defined above the polysilicon electrodes, to etch the polysilicon with very great accuracy and little spread at the location of these apertures. In addition, only a small amount of polysilicon has to be etched, so that the etch time can remain short, the thickness of the remaining polysilicon can be very accurately controlled and the spread in thickness is small. The uniformity between the pixels is improved substantially. It is possible to first etch the polysilicon through the apertures in a resist pattern above the first electrodes, remove the resist and, subsequently, using a different mask, etch the polysilicon through the apertures in a resist pattern above the second electrodes. This enables the uniformity between the pixels to be further improved. As the thickness of the thin portions of the polysilicon electrodes varies very little, the thickness of the thin portions of the electrodes can be further reduced. This is very advantageous because the quantum efficiency with which, in particular, short-wave electromagnetic radiation is converted to electric charge, increases very substantially as the thickness of the polysilicon decreases.
Preferably, the apertures in the center of the polysilicon electrodes are etched. As the second layer of polysilicon follows the topography of the first electrodes there is always a difference in thickness, as a result of the step coverage, between the polysilicon on the edges and the polysilicon in the central portion of the second electrodes to be formed. As a result of the variation in thickness of the polysilicon at the edges of the second electrodes, the best uniformity of the thin portions is achieved in the center of the electrodes.
It is cheap to form the thinner portions of the electrodes simultaneously. In this case, only one exposure step of the resist through a mask is required and an etch step to etch the apertures in the polysilicon electrodes.
By oxidizing the electrodes, the thin polysilicon portion of the electrodes can be reduced in thickness. Reducing the thickness of the polysilicon electrodes by oxidation, for example in O2, can be carried out even more accurately and uniformly than removing polysilicon by etching. In addition, oxidation is a much slower process, so that a better control of the thin portion of the polysilicon electrode is obtained.
Similarly, the thickness of the polysilicon electrodes can be reduced in a very accurate manner by nitridation. During thermal nitridation of polysilicon in, for example, N2O or NO, the polysilicon surface is very slowly consumed, thereby forming Si3N4.
Favorably, the thin portion of the gate is surrounded by substantially perpendicular walls of the thick portion of the gate. As the walls are steeper, the amount of light reflected by the walls decreases, and non-uniformities caused by adsorption differences on the walls of the different pixels are minimal.
Advantageously, the thin portion of the electrode has a maximum thickness of 50 nm. The absorption of short-wave electromagnetic radiation in the polysilicon electrode is substantially reduced below 50 nm, as a result of which the sensitivity to blue light increases substantially. The sensitivity to blue light determines the brightness of the pixels. The brightness of the pixels increases substantially as the amount of blue light absorbed in the electrodes decreases.
Advantageously, the thick portion of the polysilicon has a minimum thickness of 250 nm. A low resistance of the electrodes is advantageous because it enables the matrix to be read at a high clock rate. A low resistance of the electrodes corresponds with a low sheet resistance. The sheet resistance is defined as the specific resistance of a layer divided by the thickness of the layer. A matrix of, for example, 3 k by 2 k pixels and a clock rate of 1 MHz typically has an electrode resistance of several hundred Ohm. This corresponds to a minimum thickness of 250 nm for a properly conducting polysilicon having a specific resistance of typically 10xe2x88x924 Ohmcm.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.